When electric motors within the tool-making industry are repeatedly switched between forward and reverse, large inrush currents are temporarily generated for short time intervals. To prevent circuit interruption each time the current is reversed, fuses are generally used in series with overload relays, to protect the associated equipment from abnormal overcurrent conditions. Due to the occurrence of thermal aging when fuses are subjected to repeated high currents, the fuses must be periodically replaced. The higher the inrush current, a phenomena associated with high efficiency motors, the greater the degree of thermal aging so that higher rated fuses are required to resist thermal aging where high efficiency motors are employed. U.S. Pat. No. 5,426,406 entitled "Induction Motor Protective Circuit Breaker Unit" describes the selection of the fuse and circuit breaker for reduced thermal damage to the fuse and effective coordination with the overload relays. Because of their specialized application, the combination of such high rated fuses with overload relays find limited application outside of the tool-making industry.
U.S. Pat. No. 5,539,370 entitled "Inductive Motor Protective Circuit Breaker Unit" describes a polymer short circuit protection element that limits the short circuit current by switching from low resistance to high resistance. The polymer's resistance profile is characterized by its positive temperature coefficient of resistance (PTCR), and exhibits a sharp transition from low to high electrical resistance at a specified temperature. Repeated current reversals from tool-making operations result in cyclic heating of the polymer with little or no cooling periods. to prevent the polymer from inadvertently transitioning from low to high electrical resistance as it heats up, polymer elements having higher steady state current ratings are required. However, the higher rated elements are unable to provide optimum protection under short circuit conditions due to the increased energy required to facilitate switching.
U.S. Pat. No. 4,884,164 entitled "Molded Case Electronic Circuit interrupter" describes an industrial-rated circuit breaker having an electronic trip unit that is adjustable over a wide range of long and short time over-current conditions.
U.S. Pat. No. 4,967,304 entitled "Digital Circuit Interrupter With Electric Motor Trip Parameters" describes a digital circuit interrupter with electric motor trip parameters that includes control algorithms tailored for motor protection applications.
U.S. Pat. No. 5,614,881 filed Aug. 11, 1995, entitled "Current Limiting Device" describes a polymeric current limiting element that is not dependent on a PTCR characteristic.
It would be economically advantageous to provide a circuit breaker employing an electronic trip unit with a current suppressing unit that is not susceptible to cyclic thermal aging, and does not require periodic replacement when used in tool-making operations.
One purpose of the invention is to provide a combined circuit breaker and current suppressing unit that does not interrupt the protected circuit upon motor reversal can be adjusted to provide coordination with the thermal overload relay, and provides optimal protection for short circuit currents.